The present invention relates to a well cleaning tool for removing debris accumulated in a well casing and more specifically to such a tool having a plurality of bristles or scratching elements for removing the debris. The well cleaning tool may be used alone, but may also be used in combination with solvents and/or cleaning fluids which assist removal of the debris. Such debris typically comprises drilling fluid, cement compositions, pipe joint compositions and other materials.
Oil, gas, water and other types of wells almost always employ a steel pipe or well casing to ensure the integrity of the well borehole. The well casing is cemented during the completion stage of a drilling operation. This cementing operation inevitably leaves cement residue on the well casing wall which must be removed before initiating well production. Cleaning of the well casing is also necessary at intervals during well production to remove debris and residues, such as oil paraffin and scale which have accumulated on the well casing wall.
Well casing scrapers are well known in the art. One such known scraper comprises an elongate body or xe2x80x9cplugxe2x80x9d to which are attached metal blades. The blades scrape the casing wall as an hydraulic or mechanical force displaces the body through the casing. A disadvantage of such scrapers propelled by hydraulic force is that the scraper and the debris it removes from the well casing wall are not retrievable. Instead, the scraper and the debris are pushed through the well bore casing past the point of interest, where they remain to clutter the well borehole.
Another scraper known in the prior art is connected to a work string which mechanically drives the scraper through the well bore casing to effect a cleaning action. Once the cleaning operation is completed the scraper is removed from the well bore casing with the drill string. In GB-A-2295632 the scraper comprises an elongated, generally cylindrical member and a helical swath of scratching elements or bristles affixed to the side walls of the cylindrical member. The side walls of the cylindrical member are resiliently compressible to allow insertion of the tool into the casing and to exert a radially outward pressure on the scratching elements against the inner wall of the well casing. The side walls of the cylindrical member also define a helical channel adjacent to the swath of scratching elements such as to allow cleaning flowing to the scraper to circulate freely between the cylindrical member and the inner wall of the well casing. The cleaning fluid serves to carry away loosened debris scoured from the wall of the well casing.
The resiliently compressible side walls of the cylindrical member in which the scraper elements are mounted is comprised of a foam like material covered with a heavy fabric covering. The foam like material provides the required resiliency, whilst the fabric covering supports the scraper elements. Securing the foam and fabric covering to the cylindrical member can present problems. Moreover, the design of this scraper tool means that it is not robust enough for rough oilfield usage; it is particularly susceptible to loosing bristles.
Furthermore, because only one helical path is provided for the passage of well bore fluids around the scraper tool, the scraper tool can be subject to a so-called xe2x80x9cplungerxe2x80x9d effect as it is pushed into and pulled out of the well bore casing; that is to say pressure may build up in front of the scraper tool.
It is an object of the present invention to provide a well cleaning tool which is robust enough for oilfield usage.
It is another object of the present invention to provide a well cleaning tool in which the scratching elements or bristles can be replaced as they become worn, thus allowing repeated usage of the scraper tool.
It is a further object of the present invention to provide a well cleaning tool which allows for the free circulation of well bore fluids between it and the well bore casing and which is, therefore, subject to little or no plunger effect.
According to the present invention there is provided a well cleaning tool adapted for connection to a drill string for removing debris and residues from the inner wall of a well casing, the tool comprising an elongate generally cylindrical member having a plurality of scratching elements in the outer surface of the said member, wherein first and second helical channels or grooves are defined in the outer surface of the said member between the said scratching elements, the second helical channel or groove running in the counter direction to the first helical channel or groove.
Preferably, the scratching elements are spring steel bristles. The spring steel bristles are secured in holes in the outer surface of the cylindrical member and can be replaced as they become worn.
Preferably, the well cleaning tool comprises a generally cylindrical rigid sleeve which is releasably mounted on a mandrel adapted for connection to a drill string. The cylindrical rigid sleeve defines the said outer surface in which the scratching elements are mounted, including the first and second helical channels or grooves. Conveniently, the mandrel is threaded on both ends for connection to a drill string and has an annular bore for allowing fluid to be circulated through it.
Preferably, the cylindrical rigid sleeve is comprised of aluminium alloy.
In one embodiment of the present invention the cylindrical rigid sleeve is secured against rotation on and against longitudinal movement along the said mandrel. Conveniently, this is achieved by providing a two-part mandrel, the first part defining a pin or spindle which extends through the said cylindrical rigid sleeve and having an externally threaded section at the inner end thereof onto which an internally threaded section in one end of the cylindrical rigid sleeve is screwed, and the second part defining a locking collar which is attached to the pin or spindle at the outer end thereof to lock against the other end of the cylindrical rigid sleeve. Conveniently, the outer end of the pin or spindle defines an externally threaded section which engages with an internally threaded section in the end of the second part of the mandrel.
In a second embodiment of the present invention the cylindrical rigid sleeve is selectively rotatable on the said mandrel with the drill string, Conveniently, this is achieved by providing a dog clutch between adjacent ends of the sleeve and the mandrel which, in use, are located at the forward end of the well cleaning tool, and by allowing longitudinal movement of the sleeve relative to the mandrel to cause the dog teeth of the dog clutch to selectively engage with each other. The facility to selectively engage the cylindrical rigid sleeve with the mandrel carrying it in the drill string is particularly useful when the well cleaning tool is employed above a drill bit which is drilling out a cement plug or cement retainer bridge plug. If the sleeve is allowed to always rotate with the drill string it would soon wear out and cease to scour the inner walls of a well casing effectively. However, because the dog clutch between the sleeve and the mandrel is disengaged as the well cleaning tool travels down into the well bore (the sleeve is pushed back towards the rearward end of the mandrel) any rotation of the drill string to effect a drilling action is not translated into rotational movement of the sleeve. In contrast, once the drilling action has been completed and the well cleaning tool has been pushed down to the drilled region, the drill string can be pulled back, causing the sleeve to slide to the forward end of the mandrel and thereby engaging the dog clutch. With the dog clutch engaged the well bore casing can be cleaned and scoured as the well cleaning tool is withdrawn by both rotational and longitudinal movement of the sleeve.